The invention relates to a method for operating a particle sensor in order to determine a particle content in a gas flow, wherein the particle sensor comprises on its surface at least two interdigital IDE electrodes engaging in one another and a heating element, which is separated from the IDE electrodes by an insulation layer and by which, in a regeneration phase, the particle sensor can be heated and a soot load on the particle sensor can thereby be removed, and with which, in a diagnostic phase during the regeneration phase, a current is measured by intermittently applying a measurement voltage to the IDE electrodes and, with the aid of the profile and strength of this current as a function of time, a functional test of the particle sensor is carried out.
The invention furthermore relates to a device, in particular a control and evaluation unit, for operating the particle sensor and for carrying out the method according to the invention.
Particle sensors are currently used, for example, to monitor the soot output of internal combustion engines and for on-board diagnosis (OBD), for example for functional monitoring of particle filters. In this case, collecting resistive particle sensors are known, which evaluate a change in the electrical properties of an interdigital electrode structure due to particle deposits. Two or more electrodes may be provided, which preferably engage in one another in the manner of a comb. These are also referred to as interdigital electrodes (IDE). By an increasing number of particles deposited on the particle sensor, the electrodes are short-circuited, which leads to an electrical resistance decreasing with increasing particle deposition, a decreasing impedance or change in a characteristic quantity, such as a voltage and/or a current, associated with the resistance or the impedance. For evaluation, a threshold value, for example of a measurement current between the electrodes, is generally established and the time taken to reach the threshold value is used as a measure of the amount of particles deposited. As an alternative, a signal rate of change during the particle deposition may also be evaluated. When the particle sensor is fully loaded, the deposited particles are burnt in a regeneration phase with the aid of a heating element integrated in the particle sensor.
Such a resistive particle sensor is described in DE 101 33 384 A1. The particle sensor is constructed from two comb-like electrodes engaging in one another, which are at least partially covered with a trapping sleeve. When particles are deposited from a gas flow on the particle sensor, this leads to an evaluable change in the impedance of the particle sensor, from which the amount of particles deposited and therefore the amount of particles entrained in the exhaust gas can be deduced.
In the resistive particle sensor, the self-diagnosis of the interdigital electrodes (IDE) is based on a current measurement at elevated temperatures. Owing to the presence of sodium ions in the insulation layer under the electrode, there is in this case a certain measurable electrical conductivity. This diagnosis is therefore carried out during the sensor regeneration, during which active heating is carried out anyway and temperatures >750° C. are reached. According to the prior art, the negative IDE electrode (IDE−) is grounded during this phase, as is the positive IDE electrode (IDE+) except for the short diagnostic phase, for which reason the positive heating element terminal and parts of the heating element always have a positive electrical potential relative thereto during operation.
Since the regeneration, during which soot particles are burnt, typically lasts many seconds to minutes, positively charged particles, particularly the Na+ ions, experience a driving force from the interior of the sensor, where the heating element is located, to the surface where the IDE electrodes lie, for this prolonged time. Because of the high sensor temperature during this phase, the Na+ ions have a high mobility and migrate upward toward the surface of the particle sensor. Furthermore, on the surface and in layers near the surface, the Na+ ions experience a driving force toward the negative IDE electrode (IDE−) during phases in which a positive potential is applied to the positive IDE electrode (IDE+) and the sensor temperature is still high. This is the case during the self-diagnosis itself as well as at the start of a measurement phase, the sensor temperature being less in this case than during the regeneration.
On the surface, and to an increased extent in the vicinity of the negative IDE electrode (IDE−), concentration of the Na+ ions ultimately takes place. Exhaust gas condensate, or water, which passes through the sensor when starting the engine until the dew point end (DPE) or condenses on the electrodes during cooling of the exhaust gas system, washes out the ions or sodium compounds resulting therefrom, particularly during a cold start. Because of the subsequent temperature rise during the sensor operation, evaporation takes place, which ultimately leads to loss of the Na+ ions which are only present to a limited extent. Since, by principle, this sensor regeneration and the self-diagnosis take place periodically, and therefore often, corresponding ageing is to be expected, which may possibly lead to failure of the statutorily prescribed self-diagnosis possibility of the electrodes within the intended lifetime. Migration of the Na+ ions to the electrode, and particularly toward the negative IDE electrode (IDE−), and ultimately loss thereof over the lifetime, cannot therefore be prevented. Conversely, in the case of systems with exhaust gas systems in which Na+-rich exhaust gas condensates occur, it is also possible that Na+ ions will enter the sensor element and accumulate there. With an increasing amount, the Na+ ions cause interference. In the worst case, this can lead to detection of a shunt, and therefore to a sensor defect. Other ions, for example K+ ions, can likewise lead to the same effect and be treated correspondingly.
In a subsequent sensor generation, the sensor self-diagnosis is based no longer on the Na+ ion conduction but on electronic conductivity. This is achieved by doping the insulation layer under the IDE electrodes with iron during production of the sensor, and can be measured at comparable temperatures during operation. In this case, ions are nevertheless contained in these layers and can cause interference, particularly when ions are additionally introduced from the outside as described above.
DE 10 2009 028 239 A1 describes a method and a device for diagnosing a collecting particle sensor with a substrate, two interdigital electrodes and a heating element. The diagnosis is carried out at high temperatures by means of a current measurement between the electrodes, the flow of current taking place through a semiconducting layer arranged under the electrodes. The charge transport itself takes place by means of mobile ions, particularly by means of sodium ions, which are introduced into the substrate by impurities during the production process or by deliberate doping, and which form the semiconducting layer. In one embodiment, the diagnosis is carried out with AC voltage, which prevents polarization. When a DC voltage is used, the decrease in the conductivity due to polarization effects is detected. For regeneration, a subsequent heat treatment is provided, by which a uniform distribution of the ions is re-established. It is furthermore proposed to apply a recharging pulse lasting from 1 to 1000 ms with a voltage of about −10 V DC, in order to achieve active regeneration. The described method in this case relates to the diagnostic phase.
It is therefore an object of the invention to introduce new types of operation, or operating phases, during phases in which the particle sensor is not in the measurement mode but has elevated temperatures, i.e. particularly during the sensor regeneration, in order deliberately to displace the Na+ ions so as to prevent loss of these ions or, in the event of an excess of Na+ ions in the sensor element in the close vicinity of the IDE electrodes, to deliberately displace them away, preferably permanently, deep into the bulk or deliberately to an electrode on the surface.
It is furthermore an object of the invention to provide a corresponding device for carrying out the method, in particular a control and evaluation unit.